Non-ionizing and penetrative nature of terahertz (THz) radiation makes it promising for various detection methods in the commercial and defense industry [1-2]. Likewise, in the medical scene, particular bands in the THz frequency regime can be identified as markers of malignant tissues. Tuned to these marker frequencies, THz radiation recently has been proposed as an effective tool for cancer detection that will exhibit satisfactory resolution, substantial penetration depth, and non-harmful radiation properties in contrast to the x-ray technology. This is especially true and important for the case of breast cancer with recently identified marker frequencies of 500 and 800 GHz. According to 2006 American Cancer Society surveillance research, one out of eight women will have breast cancer in their lifetime with 96% of these cases being curable if early detected. Moreover, real-time viewing and identification of the excised tissues during medical operation is highly desired in order to decrease the biopsy time and number of follow up operations.
Medical images using THz radiation typically are generated through a mechanical raster scan of the object. However, long image acquisition times associated with such a raster scan constitute a major bottleneck. Therefore, rapid THz imaging systems based on large arrays of sensitive detectors recently have been considered within the commercial and scientific communities. In the work disclosed herein, a focal plane imaging array topology with low noise and highly sensitive heterojunction detector diodes is developed. Specifically, we consider two major needs associated with the resolution of the THz imaging arrays constructed on extended hemispherical lenses. These needs include:
(1) Compact THz detector layout for tightly packed 2D focal plane imaging arrays. For example, Schottky diodes monolithically integrated within double slot antennas were previously employed in heterodyne THz detectors settings. Although these detectors are attractive in conjunction with the double slot antennas (because of their high Gaussian beam coupling efficiency and diffraction limited patterns [3]), the need for local oscillator signal and relatively large low-pass IF filter sections do not allow for tightly packed array development.(2) Large number of antenna/detector elements (or equivalently pixels) without resorting to expensive and bulky lenses. When an extended hemispherical lens is used to focus the image on the array elements, reflections at the lens/air boundary significantly reduce coupling efficiency of the pixels positioned away from the lens axis. Therefore, the number of detector elements is significantly limited by the lens diameter, and cannot support imaging for scan angles beyond ±20° [4].
To alleviate these issues, in this disclosure, we disclose and verify a dual slot antenna element integrated with a zero-biased Sb-heterostructure backward diode for direct detection of THz radiation. In addition, we consider improved antenna layouts that can support tilted radiation patterns in order to increase the number of detectors without resorting to expensive and large silicon lenses.
A general discussion of HBD structures is set forth in U.S. Pat. No. 6,635,907. An improved version of such HBD is used in the present disclosure. In particular, the Sb-heterostructure backward diode of use in the present disclosure is an InAs/AlSb/GaSb backward diode having a p-type 6-doping plane with sheet concentration of 1×1012 cm−2 in the n-InAs cathode layer, as disclosed in the following references: N. Su, R. Rajavel, P. Deelman, J. N. Schulman, and P. Fay, “Sb-Heterostructure Millimeter-Wave Detectors With Reduced Capacitance and Noise Equivalent Power,” IEEE Electron Device Letters, vol. 29, no. 6, pp. 536-539, June 2008; Su, Zhang, Schulman, and Fay, “Temperature Dependence of High Frequency and Noise Performance of Sb-Heterostructure Millimeter-Wave Detectors,” IEEE Electron Device Letters, Vol. 28, No. 5, May 2007; Fay, Schulman, Thomas, Ill., Chow, Boegeman, and Holabird, “High-Performance Antimonide-Based Heterostruccture Backward Diodes for Millimeter-Wave Detection,” IEEE Electron Device Letters, Vol. 23, No. 10, October 2002; and WO/2010/06966 published Feb. 22, 2010 (corresponding to PCT/US09/45288 filed on May 27, 2009). The disclosures of all of these references are expressly incorporated herein by reference.
Such preferred backward diodes as referenced immediately above can be described as a “cathode layer adjacent to a first side of a non-uniform doping profile, and an Antimonide-based tunnel barrier layer adjacent to a second side of the spacer layer having monolithically integrated antennas bonded thereto”. The Antimonide-based tunnel barrier of such backward diodes may be doped. Such doping may be a non-uniform delta doping profile. This HBD, then, will be referred to herein as “a cathode layer adjacent to a first side of a non-uniform doping profile, and an Antimonide-based tunnel barrier layer adjacent to a second side of the spacer layer” for ease in discussion.